P
lanetary geology is the study of the formation and evolution of the bodies in our solar system—
including the eight planets and myriad smaller objects: moons, asteroids, comets, and mete-
oroids. Studying these objects provides valuable insights into the dynamic processes that
operate on Earth. Understanding how other atmospheres evolve helps scientists build better models
for predicting climate change. Studying tectonic processes on other planets helps us appreciate how
these complex interactions alter Earth. In addition, seeing how erosional forces work on other bodies
allows us to observe the many ways landscapes are created. Finally, the uniqueness of Earth, a body
that harbors life, is revealed through investigations of other planetary bodies.

FOCUS ON CONCEPTS
To assist you in learning the important concepts in this chapter, focus on the following questions:
! What is the nebular theory?
! What are the general characteristics that differentiate the terrestrial planets from the Jovian planets?
! What factors determine the variations observed among the atmospheres of planetary bodies?
! How did Earth’s Moon form?
! What are the major features of the lunar surface?
! How is crater density used to date surface features on the Moon?
! Why is the Venusian surface so hot?
! What surface features do Mars and Earth have in common?
! Why is the discovery of subsurface ice on Mars important?
! What are the “spots” in the atmosphere of the Jovian planets?
! Which planets have rings?
! Where are most asteroids found?
! How are asteroids different from comets?
! What are dwarf planets?

Our Solar System: respect to the Earth–Sun orbital plane, known as the ecliptic, is
shown in Table 22.1.
An Overview
The Sun is at the center of a revolving system, trillions of miles Nebular Theory: Formation
wide, consisting of eight planets, their satellites, and numerous
smaller asteroids, comets, and meteoroids (Figure 22.1). An esti- of the Solar System
mated 99.85 percent of the mass of our solar system is contained The Nebular theory, which explains the formation of the solar
within the Sun. Collectively, the planets account for most of the system, states that the Sun and planets formed from a rotating
remaining 0.15 percent. Starting from the Sun, the planets are cloud of interstellar gases (mainly hydrogen and helium) and dust
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Nep- called the solar nebula. As the solar nebula contracted due to
tune (Figure 22.1). Pluto was recently reclassified as a member of gravity, most of the material collected in the center to form the
a new class of solar system bodies called dwarf planets. hot protosun. The remaining materials formed a thick, flattened,
Tethered to the Sun by gravity, all of the planets travel in the rotating disk, within which matter gradually cooled and con-
same direction on slightly elliptical orbits (Table 22.1). Gravity densed into grains and clumps of icy, rocky material. Repeated
causes objects nearest the Sun to travel fastest. Therefore, Mer- collisions resulted in most of the material eventually collecting
cury has the highest orbital velocity, 48 kilometers per second, into asteroid-sized objects called planetesimals.
and the shortest period of revolution around the Sun, 88 Earth- The composition of planetesimals was largely determined by
days. By contrast, the distant dwarf planet Pluto has an orbital their proximity to the protosun. As you might expect, tempera-
speed of just 5 kilometers per second and requires 248 Earth-years tures were highest in the inner solar system and decreased toward
to complete one revolution. Most large bodies orbit the Sun the outer edge of the disk. Therefore, between the present orbits
approximately in the same plane. The planets’ inclination with of Mercury and Mars, the planetesimals were composed of mate-

chemical composition of planets are largely responsible for their den-
planetary debris.
rials with high melting temperatures—metals and rocky sub. Neptune’s
attract and retain large quantities of hydrogen and helium. through repeated collisions and accretion (stick-
and Atmospheres
ing together) these asteroid-sized rocky bodies combined to form
the four protoplanets that eventually became Mercury.
Earth. the present-day solar system is a much
quieter place. By comparison. Mars).
oroids. also characterized by long orbital periods and numerous satellites. which means that it would
stellar space. The planets fall into two groups based on location.
lightest elements.” A correlation exists between plan-
these planetesimals that the four outer planets eventually etary locations and sizes—the inner planets are substantially smaller
formed. For example. It was mainly from ets are known as “outer planets. ammonia. diameter of Neptune (the smallest Jovian planet) is nearly four times
sive planets. The small fraction of interplanetary matter that float if placed in a large enough tank of water. Saturn
Small bodies were flung into planet-crossing orbits. the four terrestrial
ices—water. Other properties that differ include densities. the average density of the terrestrial
the planets cleared their orbits of much of the leftover material. density of the Jovian planets is only 1. Variations in the
for the planets to gravitationally accumulate most of the inter. and mete. or into inter. Uranus. orbital periods. Venus. Furthermore. the mass is 17 times greater than that of Earth or Venus (Figure 22.2). and the Jovian (Jupiter-like) planets (Jupiter. had surface gravities sufficient to larger than the diameter of Earth or Venus. Then. Specifically.5 times that of water. contained high percentages of and Neptune).7 times that of water. tion of material resulted in the formation of three major layers
. The accumulation of ices accounts. chemical compo-
It took roughly a billion years after the protoplanets formed sitions. Our Solar System: An Overview 637
Kuiper belt
Neptune
Uranus
Asteroid Belt
Sun Mercury Venus
Earth
Mars
Jupiter
Saturn
M
V
E SUN
N U S J M
FIGURE 22. The outer planets are
escaped this violent period became asteroids. in part. Because of their relative locations. and
The planetesimals that formed beyond the orbit of Mars. the segrega-
a reduced pace. The Planets: Internal Structures
stances. has a density only 0. the terrestrial (Earth-like) planets (Mercury. and Mars. Jupiter and Saturn. for the large than the outer planets. although many of these processes continue today at Internal Structures Shortly after Earth formed.
where temperatures are low. and numbers of satellites. and methane—as well planets are also known as “inner planets” and the four Jovian plan-
as small amounts of rocky and metallic debris. whereas the average
The “scars” of this period are still evident on the Moon’s surface. The two most mas.1 Orbits of the planets. Venus. Earth. Positions of the planets are shown to scale along bottom of diagram. planets is about five times the density of water. carbon dioxide. and density. also known as gas giants. comets. This was a period of intense bombardment as sity differences. the
sizes and low densities of the outer planets. Saturn. size.

ing solar system were too hot for ices and gases to condense. iron compounds. other hydrocarbons. if any. From their centers outward. The outer cores of Earth and Mercury by charged particles of the solar wind—a necessary condition for
are liquid. the amount of hydrogen and helium increases. Jupiter and Saturn. tles. or liquid man-
ets as well. including Earth.”
metallic inner cores consisting of iron compounds at extremely By contrast. The outer cores of these two tively meager atmospheres composed of carbon dioxide. having relatively large cores its interior was hotter. fields generated by flow in their liquid outer cores. methane. have rela-
high temperatures and pressures. By
FIGURE 22. This type of chemical separation occurred in the other plan. In addition. This difference is attributable to Venus and
Mars having lower internal temperatures than those of Earth and The Atmospheres of the Planets The Jovian planets have
Mercury. mantle.4). Above their mately retained. Mars’ weak magnetic field is thought to be a remnant from when
The terrestrial planets are dense. The outer. water. Two factors explain these significant differences—solar heat-
most layers are gases and ices of hydrogen.3). while
layers differs between these two groups (Figure 22. the inner regions of the develop-
in much lower concentrations than those of Jupiter and Saturn. The Jovian atmospheres are so thick that
The two largest Jovian planets. nitro-
giants are thought to be liquid metallic hydrogen. ammo. the silicate crusts of helium. helium. whereas the cores of Venus and Mars are thought to the survival of life-forms. and All planets. the terrestrial planets. and oxygen. have significant magnetic
core. have small there is not a clear boundary between “atmosphere” and “planet. These variables deter-
nia. Silicate minerals and other lighter compounds make up very thick atmospheres composed mainly of hydrogen and
the mantles of the terrestrial planets. the nature of these solar wind and its uppermost atmosphere (ionosphere). and nickel. because the terrestrial planets are compo. Our Solar System: An Overview 639
defined by their chemical composition—the crust. Venus has a weak field due to the interaction between the
sitionally different than the Jovian planets. ammonia. and methane—which account for the low densities of these mine what planetary gases. a
the amount of metallic iron decreases while the amount of rocky planet’s magnetic field can protect its surface from bombardment
silicate minerals increase. Uranus and Neptune also have small metallic cores but during the formation of the solar system and which were ulti-
their mantles are likely hot dense water and ammonia. whereas the gen. with lesser amounts of water. However. determining the nature of a planet’s atmosphere. and
terrestrial planets are relatively thin compared to their mantles. were captured by planets
planets.
be partially molten.
mantles. except Venus and Mars.3 Comparing the internal structures of the planets. Magnetic fields play an important role in
of iron. Finally.
TERRESTRIAL PLANETS
Moon
Mercury
Mars
Key
Venus Earth
Rocky crust
Rocky mantle
Metallic core
Inner core
JOVIAN PLANETS
Uranus Neptune
Key Saturn Key
Jupiter Visible clouds Visible clouds
Gaseous hydrogen Gaseous hydrogen
Liquid hydrogen Ices (water/methane)
Metallic hydrogen Rocky core
Rocky core
. but exists During planetary formation. ing (temperature) and gravity (Figure 22.
mantles are comprised of liquid hydrogen and helium.

hydrogen and helium. and methane to condense into ices. Earth. debris (meteorites) can produce microscopic cavities on individ-
Mercury holds trace amounts of gas. however.
Solar heating (temperature)
Asteroids moons Sometime in the distant future. and car.
vapor. iest bombardment occurring 3. large impact craters are the result
The slightly larger terrestrial planets.8–4.
Saturn
Titan ending its hydrologic cycle. suggesting that it has lost as much as 90 percent of its
primitive atmosphere. the loss of hydrogen (one of
Europa
the components of water) will eventually “dry out” Earth’s oceans. Airless worlds are relatively warm and are incredibly cold. This allowed water able to retain their thick atmospheres. with the heav-
to their total mass. such as
bodies with small surface gravity.1
large quantities of the lightest gases. even the smallest pieces of interplanetary
to hold even heavy gases such as carbon dioxide and nitrogen. As a result. Following
ets probably had much thicker atmospheres. $ By what criteria are planets considered either terrestrial or
tric orbits. ual mineral grains. of collisions with massive bodies. even hydrogen and
helium move too slowly to escape the gravitational pull of these
contrast. However. the
gas giants contain large amounts of these volatiles. On bodies that have little or no atmosphere. By contrast. Early in their development. As the planets
grew.
of the coldest places in the solar system. the rate of cratering diminished dramatically and now
these primitive atmospheres gradually changed as certain gases remains essentially constant.
Gravity
the massive Jovian planets have a better chance of retaining their
FIGURE 22. are unable the Moon and Mercury. such as our Moon. For example. however. because of their great distances from
atmospheres.1 billion years ago. that period. the Jovian planets formed where temperatures were low large planets. also attracted CONCEPT CHECK 22. This was a fortuitous event % What accounts for the large density differences between the
for organisms that currently inhabit our planet. Planetary impacts were considerably more common in the
bon dioxide. which consists of fast-moving charged particles. This phenomenon occurs near the top of Earth’s atmo-
Mercury sphere where air is so tenuous that nothing stops the fastest mov-
Venus
ing ions from flying off into space. Over time. Without a
magnetic field to shield their atmospheres. This partly explains why the outer planets have been
and solar heating of planetesimals was minimal.
Because they have strong gravitational and magnetic fields. whereas others are airless include solar heating the Sun. " Briefly outline the steps in the formation of our solar system
How did Earth acquire water and other volatile gases? It seems according to the nebular theory. less massive planets have a better
Planetary Impacts
chance of losing their atmosphere because gas molecules need Planetary impacts have occurred throughout the history of the
less speed to escape their weak gravities.
which depends on the strength of the body’s gravity (see
Figure 22.
that early in the history of the solar system. our Moon. Neptune’s
have weak gravity. and Mars. at its cloud tops. Because the molecular
motion of a gas is temperature dependent. gravitational tugs by # What are planetesimals?
the developing protoplanets sent planetesimals into very eccen. For example. their
Charon Triton
upper atmospheres are exposed to the brute force of the solar
Pluto Neptune
wind. Earth’s atmosphere con-
Airless bodies
tinues to leak hydrogen and helium (the two lightest gases) into
space. Mars’ atmosphere is
enriched in heavy isotopes of both nitrogen and carbon (carbon
Bodies with an atmosphere dioxide). Furthermore. Hence.4). Jupiter and Saturn.
Mercury. the solar wind picks up
ionic gases and carries them out to space. Earth was bombarded with icy objects that Jovian?
originated beyond the orbit of Mars. Simply stated.218° C ( -360° F)—one
heating and strong gravity. ammonia. the terrestrial plan. Because weathering and erosion
.640 CHAPTER 22 Touring Our Solar System
trickled away into space. the temperatures that occur in their upper atmospheres
(temperature) and gravity. Venus. their atmospheres are miniscule compared early history of the solar system than they are today. terrestrial and Jovian planets?
& Explain why the terrestrial planets have meager atmospheres. such as asteroids and comets. nitrogen. it most easily reaches the space needed to over-
Jupiter
Galilean come Earth’s gravity. Bodies with significant atmospheres have weak atmosphere has a temperature of about .
Uranus
Because Mars and Venus lack significant magnetic fields. Because hydrogen is
Mars the lightest gas. and numerous other small bodies lack
as compared to the Jovian planets.
significant atmospheres even though they certainly would have
been bombarded by icy bodies early in their development. may still hang on in
Earth’s polar regions.4 The factors that explain why some bodies have thick atmospheres. Comparatively warm solar system.
retain some heavy gases including water vapor. the largest Jovian planets. The speed required to escape
Moon Earth
a planet’s gravity is called escape velocity. Air-
less bodies develop where solar heating exceeds a certain level. Life.

The energy of the
period of rotation on its axis equals its period of revolution around rapidly moving meteoroid is transformed into heat and compressional
Earth. Earth’s atmosphere is much less
effective in slowing large bodies—fortunately.
Compressional Meteoroid
On larger bodies. even though the Moon is a much smaller target and
has a weaker gravitational field?
# When did the solar system experience the period of heaviest
planetary impacts?
Melt Central
peak
Earth’s Moon: A Chip
Off the Old Block
The Earth–Moon system is unique because the Moon is the largest
satellite relative to its planet. All of the waves. Because its
FIGURE 22.5 Formation of an impact crater.160 miles). impacts of low-mass bodies pro-
duce only small craters on Earth.756 kilometers (7. our unique planet–satellite system
is closely related to the mechanism that created it.6. For example. such as the one in the large crater in Figure 22.2 Fractured
rock
" Why are impact craters more common on the Moon than on
Earth. have been
collected from the Moon.5 times that of water). Much of
the material expelled.
where it accumulates to form a rim. (After E. Craters excavated by
objects that are several kilometers across often exhibit a central
peak. High-speed
ejecta
The formation of a large impact crater is illustrated in
Figure 22. they make very rare
appearances. The
Moon’s surface temperature averages about 107° C (225° F) for
daylight hours and . evidence of
their cratered past is clearly evident.926 miles). Earth’s atmo-
sphere causes meteoroids with masses of less than 10 kilograms
(22 pounds) to lose up to 90 percent of their speed as they pene-
trate the atmosphere. thick atmospheres may cause the impacting wave
objects to break up and/or decelerate. the same lunar hemisphere always faces Earth.
about one-fourth of Earth’s 12. Earth’s Moon: A Chip Off the Old Block 641
are almost nonexistent on the Moon and Mercury.
The diameter of the Moon is 3. as well as rocks consist-
ing of broken fragments welded by the heat of impacts. but considerably less than Earth’s
average density (5.
The Moon’s low mass relative to Earth results in a lunar
gravitational attraction that is one-sixth that of Earth. Large meteoroids may gen-
erate sufficient heat to melt some of the impacted rock. Small secondary craters are formed by the material
The Moon’s density is 3. Shoemaker)
that of mantle rocks on Earth. A person
. Mars is the only other terrestrial
planet with moons.
CONCEPT CHECK 22.
which ejects material from the surface. The meteoroid’s high-speed impact compresses the
material it strikes. causing an almost instantaneous rebound. Samples
of glass beads produced in this manner. called ejecta. comparable to “splashed” from the impact crater. The Moon’s relatively
small iron core is thought to account for much of this difference. Therefore.475 kilometers (2. The rebound of the compressed rock causes debris to be
landings of manned Apollo missions were confined to the side of ejected from the crater.153° C ( -243° F) for night. Most of the 150 or so satellites of the Jovian planets are com. none of which resemble
the Moon. allowing planetary geologists to learn Melt
about such events. but its tiny satellites are likely captured aster. M.5. Ejecta
Uplifted blanket
oids.3 times that of water. beads. As we will see later. lands in or near the crater. crater rim
posed of low-density rock–ice mixtures. Heat melts some material. producing glass
the Moon facing Earth.

they were called maria (mar = sea. The major features are
weighing 150 pounds on Earth weighs only 25 the dark maria and the light. These vast plains
are strongly concentrated on the side of the Moon facing Earth
and cover about 16 percent of the lunar surface. Clearly visible are the bright
rays. The
Apollo 11 mission showed conclusively that the maria are exceed-
ingly smooth plains composed of basaltic lavas. Current models Mare Tranquillitatus
show that Earth is too small to have formed with a Copernicus (Sea of Tranquility
moon. The Moon’s small mass (and
low gravity) is the primary reason it was not able
to retain an atmosphere. a Kepler crater
captured moon would likely have an eccentric crater
orbit similar to the captured moons that orbit the
Jovian planets. particularly one so large.7). These areas are now generally referred to as the lunar
highlands because they are elevated several kilometers above the
maria. highly cratered highlands. resembling seas on
Central peak Earth. secondary craters.6 The 20-kilometer-wide lunar crater Euler in the The arrangement of terrae and maria result in the legendary
southwestern part of Mare Imbrium. although their mass
remains the same. Rocks retrieved from the highlands are mainly breccias. an
astronaut could jump six times higher on the
Moon than on Earth.
pulverized by massive bombardment early in the Moon’s history. and the large accumulation of
“face” of the “man in the moon. The lack of large
volcanic cones on these surfaces is evidence of high eruption rates
Continuous ejecta of very fluid basaltic lavas similar to the Columbia Plateau flood
basalts on Earth. some of the ejected debris was
thrown into Earth’s orbit and gradually coalesced
to form the Moon. (Collisions of this type
were probably frequent at that time. which consists of a
Crater ray large mantle and a small iron-rich core. he observed two different types of terrain: dark
chain lowlands and brighter. semi-molten Earth
about 4. Computer simulations show
that most of the ejected material would have come
. singular mare). the Moon’s light-colored areas resemble Earth’s
continents. Mare Imbrium
(Sea of Rains)
How Did the Moon Form?
Until recently. (Courtesy of NASA)
FIGURE 22.5 billion years ago.
Discontinuous ejecta
By contrast.
Because the dark regions appeared smooth.) During this
explosive event. the origin of the Moon—our near-
est planetary neighbor—was a topic of consider-
able debate among scientists.7 Telescopic view of the lunar surface from Earth. Furthermore.
FIGURE 22. highly cratered highlands (Figure 22. (UCO/Lick Observatory Image)
pounds on the Moon. central peak.
The current consensus is that the Moon Lunar
formed as the result of a collision between a Mars. Highlands
sized body and a youthful.”
ejecta near the crater rim. so the first observers dubbed them terrae (Latin for
“land”). This impact model is consistent with the
Moon’s low density and internal structure.
The Lunar Surface When Galileo first pointed his telescope
Secondary crater toward the Moon. 642 CHAPTER 22 Touring Our Solar System
from the rocky mantle of the impactor while its core became part
of the growing Earth. If not burdened with such a load. This difference allows an astro-
naut to carry a heavy life-support system with rel-
ative ease.

the processes of weath-
years ago. such as Kepler and
Copernicus (32 and 93 kilometers in
diameter. as well as the rest of the solar system. perhaps derived from partial melting deep
within the lunar mantle. such basins make up the lunar maria and a few similar large
the number of craters per unit area.0 billion and 3. ceased. Today.7. considerably younger than the (micrometeorites) that continually bombard its surface and grad-
initial lunar crust. lava channels.8 billion atmosphere or flowing water. Filling of the impact area with fluid basalts. sank. which is composed mainly of would have long since obliterated it.
The greater the crater density. unconsolidated debris derived from a few billion years of mete-
ments. after the Moon coalesced
it passed through the following four phases: (1) formation of the that some mare-forming eruptions may have occurred as recently
original crust and lunar highlands.4 billion years ago. breccia. The larger craters shown in
Figure 22.
between 200 and 400 kilometers. while less dense Figure 22. rilles formed by localized
was most likely completely melted—literally a magma ocean. Material ejected from these younger depressions
silicate minerals floated to form the Moon’s primitive crust. A. about 3. They were likely generated by a Both the maria and terrae are mantled with a layer of gray.9). Recent evidence suggests composed of igneous rocks.
History of the Lunar Surface The A large impact basin
as it appears today with
evidence used to unravel the history A. This soil-like layer. the lunar crust was continually impacted as the Today’s Lunar Surface: Weathering and Erosion The
Moon swept up debris from the solar nebula.7. the Moon’s outer shell ter). The blankets the maria surfaces and many older. expe. and (4) formation of rayed Other lunar surface features related to this period of volcan-
craters. about 4. fluid lava
features are impact craters. the Moon. Impact of an asteroid-size mass
returned from Apollo missions and produced a huge crater hundreds of kilometers in diameter and disturbed the lunar crust far beyond
studies of crater densities—counting the crater. A mete. During this time. Partial melting probably occurred in several isolated pock. These two
craters are thought to be relatively
young because of the bright rays
(lightly colored ejected material) that
radiate from them for hundreds of
kilometers. (3) filling of mare basins. rayless craters. oric bombardment (Figure 22. Most exemplified by the 90-kilometer-wide Copernicus crater shown in
of the dense minerals. weathering and erosion
highland rock type is anorthosite. ering and erosion that continually modify Earth’s surface are
rienced a sudden drop in the rate of meteoritic bombardment. The
highlands are made of these igneous rocks that rose buoyantly relatively young Copernicus crater is thought to be about 1 bil-
like “scum” from the crystallizing magma.8). tectonic forces are no longer
The Moon’s next major event was the filling of the large impact active on the Moon. respectively) were created
from bombardment by bodies 1 kilo. In addition. so quakes and volcanic eruptions have
basins created at least 300 million years earlier (Figure 22.
of the lunar surface comes primarily
from radiometric dating of rocks FIGURE 22. absent on the Moon. and fine lunar
.8 Formation and filling of large impact basins. ually smooth the landscape. ism include small shield volcanoes (8–12 kilometers in diame-
During the late stages of its accretion. The most common lion years old.
impact basins.
Once formed.
calcium-rich plagioglase feldspar. Excavation of large begins filling the basin
oroid 3 meters (10 feet) in diameter impact basin Ejected
debris
can blast out a crater 50 times larger. Had it formed on Earth. B.
Then. B. lithos = stone). Then. C. evidence of pyroclastic eruptions.
meter or more in diameter. olivine and pyroxene. This activity has crushed and repeat-
The Mare basalts are thought to have originated at depths edly mixed the upper portions of the lunar crust. as
and underwent magmatic differentiation (see Chapter 3). Earth’s Moon: A Chip Off the Old Block 643
Some of the most obvious lunar Much later. the
older the feature. Therefore. (2) excavation of the large as 1 billion years ago. the magma ocean began to cool The last prominent features to form were rayed craters. and grabens.5 billion years. its cratered surface
C. Because the Moon is unprotected by an atmosphere. glass beads. structures on Mercury.
slow rise in temperature attributed to the decay of radioactive ele. is
retrieved during the Apollo missions.
or about 150 meters (500 feet) in
diameter. properly
ets as indicated by the diverse chemical makeup of the rocks called lunar regolith (rhegos = blanket. ero-
Radiometric dating of the maria basalts puts their age between sion is dominated by the impact of tiny particles from space
3. Moon’s small mass and low gravity account for its lack of an
several large impact basins were created. Such evidence sug-
gests that.

meters in diameter) known impact crater on Mercury is Caloris
Basin. ices that vaporized during a
recent comet impact.
planet’s interior to have already cooled. the innermost and smallest planet. and is followed by the same Venus: The Veiled Planet
duration of daylight. no sustained atmosphere. which lasts 176 Earth-days.
& List the major stages in the development of the modern lunar Mercury resembles Earth’s Moon in that it has very low reflec-
surface. these
smooth plains appear to be similar in origin to lunar maria.
Most of these smooth areas are associated with large impact
Earth’s Place in the Universe
basins. Mercury has the greatest temperature Venus. tivity. reflect-
depending on the age of the surface. hot enough to melt tin Sun in a nearly perfect circle once every 225 Earth-days.” (Courtesy of NASA)
dust. numerous volcanic features. including Caloris Basin. and
' Compare and contrast the processes of weathering and ero. named for the Roman “Goddess of Love and Beauty. data gathered by Messenger during its orbit around Mercury
Mercury.
" Briefly describe the origin of the Moon. revolves around the in 2011 will shed additional light on the relationship between cra-
Sun quickly (88 days) but rotates slowly on its axis. The minuscule amount of gas
present on Mercury may have originated from several sources:
CONCEPT CHECK 22. The largest (1. a heavily cratered terrain (Figure 22.
day–night cycle. Also like our Moon. The lunar regolith is anywhere from 2 to 20 meters thick Mercury absorbs most of the sunlight that strikes it. Mercury’s tering and volcanism. Hope-
Mercury: The Innermost Planet fully.300 kilo-
sion on Earth with the same processes on the Moon. Venus rotates in the opposite direction of the other planets
impossible on Mercury.FIGURE 22. This finding suggests that Mercury has
% How is crater density used in the relative dating of surface fea. is
extremes. second only to the Moon in brilliance in the night sky. Consequently. Notice the footprint (inset) in the lunar “soil. (retrograde motion) at an agonizingly slow pace—one Venus day
644
. the Messenger spacecraft
$ How are maria on the Moon similar to the Columbia Plateau
in the Pacific Northwest? detected a magnetic field. and/or outgassing of the planet’s interior.” It orbits the
time temperatures exceed 427° C (800° F). Mercury has smooth plains
that cover nearly 40 percent of the area imaged by Mariner 10. Images and other data gathered by Mariner 10 show evi-
dence of volcanism in and around Caloris Basin and a few other
Terrestrial Planets smaller basins. ing only 6 percent into space. which drop as low as . where lava partially filled the
! A Brief Tour of the Planets
basins and the surrounding lowlands. is very long com-
pared to Earth’s 24-hour cycle.9 Astronaut Harrison Schmitt sampling the lunar surface. a large core that remains hot and fluid enough to generate a mag-
tures on the Moon? netic field.
Although Mercury is small and scientists expected the
# Compare and contrast Moon’s maria and highlands. These extreme temperatures make life “as we know it” ever.173° C (. a characteristic of terrestrial bodies
that have little or no atmosphere.10). How-
and lead.280° F) whereas noon. One “night” on Mercury is roughly
equivalent to 3 months on Earth.3 ionized gas emitted from the Sun.

12). some of which traveled along lava
channels that extend for hundreds of kilometers (Figure 22. and incinerating most of the small debris.5 kilometers high and 400 kilometers wide. do not appear to have contributed to the
present Venusian topography.11 This global view of the surface of Venus is computer-generated from years
As a consequence. who expected that Venus would show
evidence of extensive cratering from the heavy
. The rises are
thought to have formed where hot mantle plumes encountered
the base of the planet’s crust. is about 9 kilo-
meters high and only 120 kilometers wide. Venus’ extreme conditions result in
volcanoes that tend to be shorter and wider than those on Earth
or Mars (Figure 22. abundant volcanism is associated with mantle
is equivalent to about 244 Earth days. Terrestrial Planets 645
bombardment period. the surface temperature of of investigations culminating with the Magellan mission. By com-
parison. (Courtesy of NASA/JPL)
ature variations at the surface are generally min-
imal because of the intense mixing within the
planet’s dense atmosphere. consisting mostly of carbon dioxide (97%)—
the prototype for an extreme greenhouse effect. Much like mantle
plumes on Earth. all of the probes were crushed by the
planet’s immense atmospheric pressure within
an hour of landing.
About 80 percent of the Venusian surface consists of lowlying
plains covered by lava flows.
is about 8.
Approximately 1. However. despite extreme temperatures and pres-
sures.
More than 100 large volcanoes have been identified on Venus. Mauna Loa.000 impact craters have
been identified on Venus—far fewer than Mer-
cury and Mars.
Venus’ Baltis Vallis. ridges. causing uplift.
Moon. Temper. Maat Mons. Its thick atmos-
phere also limits the number of impacts by breaking up large
incoming meteoroids.
FIGURE 22. the largest volcano on Venus. In the
1970s. that cross the globe are highly fractured ridges and canyons of the eastern Aphrodite
highland. FIGURE 22. This view of Mercury looks similar to Earth’s Venus also has major highlands that consist of plateaus. four Russian spacecraft landed success-
fully and obtained surface images. (As expected.11).
However. which recycle rigid litho-
sphere.800 kilometers (4. high surface temperatures and pressures inhibit explo-
sive volcanism. the largest volcano on Earth.) Using radar imaging. (Courtesy of NASA) and topographic rises that stand above the plains. meanders 6.255 miles) across the planet. Venus has
the densest atmosphere of the terrestrial plan-
ets.
The composition of the Venusian interior is
probably similar to Earth’s.
however.10 Mercury. but the processes
of plate tectonics. the
unmanned spacecraft Magellan mapped Venus’
surface in amazing detail (Figure 22.13). Investigations of the
extreme and uniform surface temperatures led
scientists to more fully understand how the
greenhouse effect operates on Earth. The twisting bright features
Venus averages about 450° C (900° F).
The surface of Venus is completely hidden
from view by a thick cloud layer composed
mainly of tiny sulfuric acid droplets. found instead that a period of extensive
volcanism was responsible for resurfacing Venus. Mantle convection is
thought to operate on Venus. but more than Earth. Re-
searchers. Venus’
weak magnetic field means its internal dynam-
ics must be very different. the longest known lava channel in the solar
system. In addition.

which lies approximately 300 kilometers (186
miles) west of the scene.
About two-thirds of the surface of Mars consists of heavily
cratered highlands. The red-
dish color of the Martian landscape
is iron oxide (rust). The bright areas in the foreground are surrounding some Martian craters has a different appearance—
lava flows. Martian highlands are
similar in age to the lunar highlands. has rough surfaces. The
flows that orignated from a volcano named Ammavaru. Visible on these plains are volcanic cones.000-square-kilometer pool.14).
Based on relatively low crater counts. Maat Mons.
Topography Mars. the northern plains. about 10 kilometers high.140° C (-220° F) in the winter to
highs of 20° C (68° F) in the summer. like the Moon.13 Venus’ Sapas Mons (center) is a broad volcano 400 that surrounding lunar craters is to be expected. appears to
.
as it did in the rest of the solar system. oxygen. which appears bright in this radar image.
albeit on a much smaller scale.12 Extensive lava flows on Venus. The plains’ relatively
flat topography is consistent with vast outpourings of fluid basaltic
lavas. Recent data col-
lected by the European Space Agency’s
Venus Express suggest that Venus’
highlands contain silica-rich granitic
rock. is in the background.
Located along the Martian equator is an enormous elevated
region. Plane-
This computer-generated view is constructed from data acquired by tary geologists believe that a layer of permafrost (frozen.
which account for the remaining one-third of the planet. revolves around
the Sun in 687 Earth-days. about the size of North America.
smaller craters are usually filled with
whereas the darker flows are smooth. the lava wind-blown dust—confirming that
collected in a 100. icy soil)
the Magellan spacecraft. these elevated land-
masses resemble Earth’s continents.646 CHAPTER 22 Touring Our Solar System
upwelling on Venus. called the Tharsis bulge
(Figure 22. (Courtesy of NASA/JPL)
lies below portions of the Martian surface and that impacts heated
and melted the ice to produce the fluid-like appearance of these
ejecta.
Although seasonal temperature vari-
ations are similar to Earth’s. concentrated mostly in its southern hemi-
sphere (Figure 22. Thus. Another large volcano. The lava. (Courtesy of NASA) Mars is a dry. are
younger than the highlands (Figure 22. approximately one-half the
diameter of Earth. The period of extreme cratering occurred
early in the planet’s history and ended about 3. Large impact craters provide information
about the nature of the Martian surface.
FIGURE 22. daily
temperature variations are greater
due to its very thin atmosphere (only
1% as dense as Earth’s). ejecta similar to
FIGURE 22. Upon breaking through the ridge belt (left of center). This feature.
Mars: The Red Planet
Mars. if the sur-
face is composed of dry dust and rocky debris. with
small amounts of nitrogen.14). that of a muddy slurry that was splashed from the crater. desert world.8 billion years ago. The tenuous
Martian atmosphere consists pri-
marily of carbon dioxide (95%). As such. some with sum-
mit pits and lava flows with wrinkled edges. For example. But the ejecta
kilometers (250 miles) wide.
and water vapor. Mean sur-
face temperatures range from lows of
.14). This Magellan radar image shows a system of lava is pitted with impact craters.

like the Hawaiian
Islands. Consequently. including the largest.300 kilometers (1. moving plates keep the crust in
. The main canyon is more Mars that covers an area about the size of the state of Arizona. Olympus
Mons. with
Hellas
winds up to 270 kilometers (170
Argyre miles) per hour. on Mars.
The scarcity of impact craters on some volcanic surfaces suggests
that the planet is still active. considerable evidence indicates that in the
The tectonic forces that created the Tharsis region also pro. Along the eastern flanks of the bulge. This canyon network was largely created by down-
faulting. enormous volcanoes such as
Tharsis bulge Olympus Mons form rather than a
Cratered string of smaller ones.
FIGURE 22. A regions it forms small permanent ice caps along with carbon diox-
much smaller volcanic center (bulge) also exists.S. the largest identifiable impact structure on
the planet.400 miles) in diameter
and is the planet’s lowest elevation (Figure 22.14. like spokes on a bicy. As
a result. In addition.
Volcanism was prevalent on Mars during most of its history. Once formed.14 Two computer-generated globes of Mars with some major surface features labeled. creating stream valleys and related features. In the polar
volcanic rock that includes the planet’s five largest volcanoes. (Courtesy of NASA/JPL) water does not appear to exist any-
where on the Martian surface. a series of vast One location where running water was involved in carving val-
canyons called Valles Marineris (Mariner Valleys) developed. Other buried crater basins that are even larger than Hellas
probably exist. plate
Elysium Mons tectonics is absent so successive erup-
Olympus Mons tions build up in the same location.
Color represents height above (or below) the mean planetary radius—white is about 12 kilometers Yes. Mars has several of the largest known
volcanoes in the solar system.15).14). man-
Northern tle plumes tend to produce a chain of
lava plains volcanic structures. Extensive dust storms. This gigantic volcano was last
active about 100 million years ago and resembles Earth’s Hawai-
ian shield volcanoes (Figure 22.14). with abundant dunes and
Altitude (km) low areas partially filled with dust. Water Ice on Mars! Liquid
above average and dark blue 8 kilometers below average. Most of the Martian land-
scape resembles Earth’s rocky
–8 –4 0 4 8 12 deserts. Debris ejected
from this basin contributed to the elevation of the adjacent high-
lands. which is about the size of Arizona and stands nearly three
times higher than Mount Everest.
than 5000 kilometers long. Hellas. first billion years of the planet’s history. Terrestrial Planets 647
constant motion.
highlands Currently.
cle wheel. How-
ever. Researchers have proposed that melting of subsurface
Figure 22. 7 kilometers deep.15 Image of Olympus Mons. ide ice. not by stream erosion as is the case for Arizona’s Grand
Canyon. an inactive shield volcano on
erosion and collapse of the rift walls. leys can be seen in the Mars Reconnaissance Orbiter image in
Valles Marineris is so vast it can be seen in the image of Mars in Figure 22. surface.
Other prominent features on the Martian landscape are large
impact basins.16. Thus. liquid water flowed on the
duced fractures that radiate from its center. By contrast. Valles Marineris grew by water
FIGURE 22. poleward of about 30 degrees
have been uplifted and capped with a massive accumulation of latitude ice can be found within a meter of the surface. the dominant force
shaping the Martian surface is wind
Valles Marineris erosion. Geological Survey)
wide (Figure 22.
How did the volcanoes on Mars grow so much larger than sim-
ilar structures on Earth? The largest volcanoes on the terrestrial
planets tend to form where plumes of hot rock rise from deep with-
in their interiors. can persist for weeks.
Dust devils have also been pho-
tographed. is about 2. On Earth. and 100 kilometers (Courtesy of the U. it consists of graben-like valleys similar to the East
African Rift Valleys.

the Opportunity rover investigated structures similar to
features created by water on Earth.” How are these
chaotic topography that appear to have formed when the sur. pressure could
' What evidence suggests that Mars had an active hydrologic
mount until a catastrophic release occurred.
Other channels have stream-like banks and contain numer. If the melt water
the largest ones on Mars?
was trapped beneath a thick layer of permafrost.16 This image was obtained by the Mars Reconnaissance
Orbiter and shows gullies emanating from rocky cliffs.4
ous teardrop-shaped islands (Figure 22. These included layered sedi-
Jovian Planets
mentary rocks. playas (salt flats). CONCEPT CHECK 22. and lake beds. (Courtesy of NASA/JPL)
once had flowing water. The
meandering and braided patterns are typical of water-carved
FIGURE 22.17 Stream-like channels are strong evidence that Mars
channels. satellites. creating the
chaotic terrain. with the exception of the polar regions. two planets similar? How do they differ?
face collapsed. The most likely source of water for these flood. Most of these large flood channels emerge from areas of $ Venus was once referred to as “Earth’s twin. Inset shows close-up of streamlined island
where running water encountered resistant material along its
channel. dubbed “blueberries. (Courtesy of NASA/JPL)
ice caused spring-like seeps to emerge along the valley wall. In addi-
tion. Jupiter has a mass two and a half times
of Mars for more than a billion years.”
were found that probably precipitated from water to form lake
sediments.000 times greater than those of the Mississippi than on Earth?
River. Minerals that Earth’s Place in the Universe
form only in the presence of water such as hydrated sulfates were
! A Brief Tour of the Planets
also detected. the overlying surface would collapse. These valleys appear " What body in our solar system is most like Mercury?
to have been cut by catastrophic floods with discharge rates that # Why are the surface temperatures so much higher on Venus
were more than 1. Jupiter: Lord of the Heavens
water does not appear to have significantly altered the topography The giant among planets. % What surface features do Mars and Earth have in common?
& Why are the largest volcanoes on Earth so much smaller than
created valleys was the melting of subsurface ice. tree-like pat-
terns that resemble dendritic drainage networks on Earth.
Not all Martian valleys appear to be the result of water
released in this manner. greater than the combined mass of all other planets. slowly
creating the gullies—a process that may still be active today. Small spheres of hematite.FIGURE 22.
648
. As the water cycle in the past?
escaped.17). Some exhibit branching. Nevertheless.

is a layer of brown to sinking and warming. along with the rapid rotation of the planet. The bulge of the equatorial region on Earth. and lowest. generates
the high-speed winds observed between the belts and zones.
FIGURE 22. However. Near the top of its atmosphere lie
to the Sun. which are driven by solar energy. the heat emanating
and the contraction of the polar dimension are evident (see the from Jupiter’s interior produces the huge convection currents
Polar Flattening column in Table 22. observed between the belts and zones. The areas of light clouds (zones) are regions where gases are ascending and cooling. This convective circulation. completing one rotation in timeters each year. This convective circulation. white wispy clouds of ammonia ice. east–west flow
colors are thought to be by-products of chemical reactions occur. Thus.18 The structure of Jupiter’s atmosphere. whereas the dark belts represent cool material that is
little higher.18). along with
orange-brown clouds of ammonium hydrosulfide droplets. and rotates Because of its immense gravity.18. where temperatures are cooler.1). generates the high-speed. This contraction generates most of the heat
slightly less than 10 hours. layer is composed mainly of water ice and clouds (zones) are regions where warm material is ascending and
appears blue-gray—generally not seen in visible-light images. These Jupiter’s rapid rotation.
Beltsuds)
lo
(dark c
ds
g win
Stron
s
Zoncelouds)
t
(brigh
ds
g win
Stron
s
Beltlouds)
c
(dark
North polar region
North temperate zone
North temperate belt
North tropical zone
North equitorial belt
Equitorial zone
South equitorial belt
South tropical zone
South temperate belt
South temperate zone
South polar region
. When viewed telescopically.
Jupiter orbits the Sun once every 12 Earth-years. the effect that drives Jupiter’s atmospheric circulation. A cooling.
Jupiter’s appearance is mainly attributable to the colors of Jupiter’s convective flow produces alternating dark-colored
light reflected from its three main cloud layers (Figure 22. it pales in comparison ring in Jupiter’s atmosphere. with only 1/800 of the Sun’s mass. unlike winds
of this fast spin is noticeable. Jovian Planets 649
and asteroids in the solar system. Jupiter is shrinking a few cen-
more rapidly than any other planet. The light
warmest.
Sinking dominates the flow in the darker cloud layers (belts). as shown in Figure 22. The belts and light-colored zones. observed in its atmosphere.

For instance. More recently. that that it probably had abundant have a warm. liquid water at some point in its perhaps an ocean. In all. Moreover. surfaces of some of Jupiter’s an orbiter to Europa—and even-
The innermost of the Galilean moons. Besides Earth. implying a different evo. much like
Earth’s moon. than Earth are considered too Europa’s icy surface is quite
oids or comets that passed near enough to be gravitationally cap. This suggests that
telescope and are interesting in their own right. consisting of 63
moons discovered thus far. sulfurous volcanic centers have been discovered. Io
D. finding liquid water within our as we know it. the largest Jovian satellite. D. lava. or are remnants of the collisions of larger bodies.. and have eccentric orbits steeply The planets closer to the Sun Galileo have revealed that
inclined to the Jovian equator. Callisto
C. This gravitational flexing of Io is transformed
at lower levels in the atmosphere.. The brown storm clouds reside ther from.
strikes appear to be less frequent than on Earth. thought to be mainly composed of silicate min-
erals. is densely cratered. rine—to determine if it harbors
Umbrella-shaped plumes have been observed rising from Io’s sur. den under Europa’s outer cover. The best prospects of uid water is a necessity for life
tion of each satellite is strikingly different. warm to contain liquid water.20A). For example.19 Jupiter’s four largest moons (from left to right) are called the Galilean moons because they were discovered by Galileo. resembles a miniature solar system. the outermost of the Galilean satellites. regularly erupts on its surface (Figure 22. This heat source for volcanic activity is tidal energy generated by a
enormous.19). Callisto. smallest of the Galilean moons. Europa must
Voyagers 1 and 2 revealed. are roughly the size of Mercury. Jupiter’s Rings One of the surprising aspects of the Voyager 1
Galileo discovered the four largest satellites.20B). and those farther from the Sun apparently filled with dark fluid
The Galilean moons can be observed with binoculars or a small are generally too cold (although from below. there satellites—with Io as the rope. whereas the two smaller
ones. and areas covered by numerous parallel grooves.
face to heights approaching 200 kilometers (Figure 22.
satellites. The eight
largest moons appear to have formed around Jupiter as the solar Students Sometimes Ask. Jupiter. history). Io. in 1610 (Figure 22. Io. and the other nearby satellites pull and push on Io’s tidal bulge as
The white ovals are the cold cloud tops of huge storms many times its slightly eccentric orbit takes it alternately closer to. Because liq-
The Galileo mission also unexpectedly revealed that the composi. Ganymede and
Callisto. In addition to the Great Red Spot. is one of only three volcanically active bodies known to exist in the solar system.18). but the metal) and results in Io’s spectacular sulfurous volcanic eruptions. Images from some features on Mars indicate under its icy shell. C. to the surprise of most geoscientists. (Courtesy of NASA/NGS Image Collection)
B.
Jupiter’s Moons Jupiter’s satellite system. B. Europa and Io. A. Ganymede.
system condensed. Lightning in various white oval into heat (similar to the back-and-forth bending of a piece of sheet
storms has been photographed by the Cassini spacecraft. smooth
regions. is perhaps the most moons. are about the size of Earth’s Moon. do any other bodies in the solar system
ters in diameter) that revolve in the opposite direction (retrograde have liquid water?
motion) of the largest moons. Europa. The gravitational field of Jupiter
are various white and brown oval-shaped storms (Figure 22. Ganymede
. Detailed images from life. anticylonic storm that is twice the size of Earth has relentless “tug of war” between Jupiter and the other Galilean
been known for 300 years. there has been
lution for each. young and exhibits cracks
tured by Jupiter.
Jupiter also has many very small satellites (about 20 kilome. then far-
larger than hurricanes on Earth. The
FIGURE 22. exhibits cratered areas. The
innermost moon. The two largest. more than 80 of liquid water is possibly hid. referred to as Galilean mission was the discovery of Jupiter’s ring system. Ganymede has a dynamic core that solar system lie beneath the icy considerable interest in sending
generates a strong magnetic field not observed in other satellites.19). mobile interior—
each of the four Galilean satellites is a unique world (Figure 22. Europa
A. launching a robotic subma-
active.
has an icy surface that is criss-crossed by many linear features.650 CHAPTER 22 Touring Our Solar System
The largest storm on the planet is the Great Red Spot. ing of ice. These satellites appear to be aster. an ocean tually a lander capable of
volcanically active body in our solar system.

and the atmospheric composition is about 98 percent nitrogen
and 2 percent methane with trace organic compounds.23).22). Titan. two small moons gant planet” in the early 1600s.
plume of volcanic gases and debris is rising more than 100 kilometers Enceladus is another unique satellite of Saturn—one of the
(60 miles) above Io’s surface. and internal structures are remarkably
similar. and Rhea may have its own
rings (Figure 22. surface age.
Saturn’s atmosphere. the nuclear-pow-
in size to smoke particles.20 A volcanic eruption on Jupiter’s Moon Io. Many of Saturn’s smallest moons have irreg-
ular shapes and are only a few tens of kilometers in diameter. More information was collected about Saturn in that short
The main ring is composed of particles believed to be fragments time than had been acquired since Galileo first viewed this “ele-
blasted from the surfaces of Metis and Adrastea. when the positions of Earth
and Saturn allowed the rings to be viewed edge-on. such
as dune formation and stream-like erosion caused by methane
“rain. the Hubble Space Telescope. the
atmosphere’s dynamics are driven by the heat released by gravi-
tational compression.
sion. The volcanic-
like activity occurs in an area called “tiger stripes” that consists
the ring system was thoroughly investigated by the Galileo mis. B. is larger than Mercury and is
the second-largest satellite in the solar system.
Saturn’s Moons The Saturnian satellite system consists of 61
known moons of which 53 have been named.
the clouds are composed of ammonia. Cassini-Huygens spacecraft have added to our knowledge of Sat-
samer ring formed. and Nep-
tune’s Triton. The bright red area on the left side of few where active eruptions have been observed (Figure 22. More recently. yet their atmo.000 miles of its
rings indicates that these minute particles are widely dispersed. Saturn’s ring system is more like a large rotating disk of vary-
urn is almost twice as far from the Sun as Jupiter. Titan was visited and pho-
tographed by the Huygens probe in 2005. By analyzing how these rings scatter light. compositions. Jovian Planets 651
A.
first observed by Galileo in 1610 (Figure 22. like Jupiter’s.5 times that at Earth’s surface. Furthermore. surface. and origin. where internal forces have ripped apart
their icy surfaces. Like Jupiter.” In addition. The atmospheric pres-
sure at Titan’s surface is about 1. Others. similar Saturn’s Ring System In the early 1980s. shape. are the only satellites in the solar system known to
have substantial atmospheres. the northern latitudes appear to have lakes of
FIGURE 22. Their ring nature was determined 50 years later by Dutch
astronomer Christian Huygens. researchers deter-
mined that the rings are composed of fine.
Saturn’s largest moon. is dynamic (Figure 22. Titan. Although the
atmosphere is nearly 75 percent hydrogen and 25 percent helium. Impacts on Jupiter’s moons Amalthea and Thebe are ground-based telescopes.
Although the bands of clouds are fainter and wider near the equa-
tor.21). A. Saturn’s
faintest rings and satellites became visible. rotating “storms” similar to Jupiter’s Great Red Spot occur in
Saturn’s atmosphere. dence of tectonic activity. comprised mostly of water. ammonia hydrosulfide. each segregated by temperature.
.)
Requiring more than 29 Earth-years to make one revolution. of four large fractures with ridges on either side. dark particles. urn’s ring system.
the image is newly erupted lava. The most striking feature of Saturn is its system of rings.21). In 1995 and 1996. At least two (Dione and Tethys) show evi-
B. Twenty-three
of the moons are “original” satellites that formed in tandem with
their parent planet. With his primitive
telescope. like Hyperion. as does intense lightning. spheres. observations from
of Jupiter. (Courtesy of NASA) The outgassing. ing density and brightness than a series of independent ringlets. The moons vary
significantly in size. Sat. is thought to be the
source replenishing the material in Saturn’s E ring. This
liquid methane. (The rings were visi-
Saturn: The Elegant Planet ble edge-on again in 2009. the faint nature of the ered Voyagers 1 and 2 explored Saturn within 100. and the
believed to be the source of the debris from which the outer Gos.
and water. the rings appeared as two small bodies adjacent to the
planet. Titan has
Earth-like geological landforms and geological processes. are so porous that
impacts punch into their surfaces.

(Courtesy of
from top to bottom. It seems. particles collide frequently
as they orbit the planet. designated A and B. Recall that volcanic-like activity
on Saturn’s satellite Enceladus is thought to be the source of mate-
rial for the E ring. imaged are 40.
The origin of planetary ring systems is still being debated.21). Or. There are only a few gaps—most tightly packed and contain particles that range in size from a few
of the areas that look like empty space either contain fine dust par. most of the particles being roughly the size of a large snowball
(see Figure 22. Saturn’s outer-
most ring (E ring). with
ticles. centimeters (pebble-size) to tens of meters (house-size).
that planetary rings are not the timeless features that we once
thought—rather. Hyperion. Mimas. tiny particles.22 Saturn’s impact-pummeled satellite.652 CHAPTER 22 Touring Our Solar System
Cassini’s division
Encke gap
A
B
Saturn D C
FIGURE 22.
arises from the gravitational pull of Jupiter’s moon. perhaps the rings
. Per-
haps the rings formed simultaneously and from the same material
as the planets and moons—condensing from a flattened cloud of
dust and gases that encircled the parent planet.21. only 10–30 meters
by the Cassini Orbiter.
Studies have shown that the gravitational tugs of nearby moons
tend to shepherd the ring particles by gravitationally altering their
orbits (Figure 22.
called A ring (outer) and B ring (inner). (Courtesy of NASA)
Each ring is composed of individual particles—mainly water ice Most of Saturn’s rings fall into one of two categories based on
with lesser amounts of rocky debris—that circle the planet while density. a clearly visible gap in Figure 22. are separated by the Cassini division. are
regularly impacting one another.000 kilometers wide. Although Saturn’s main rings (A and B)
FIGURE 22. which is very nar-
row.
NASA/JPL) At the other extreme are Saturn’s faint rings. It is also possible that mate-
rial is continually recycled between the rings and the ring moons. the F ring. In the dense rings. For example.
Whereas the Cassini Division. they are continually recycled.24).21. Saturn’s main (bright) rings. is composed of
widely dispersed. appears to be the work of satellites located on either side that
confine the ring by pulling back particles that try to escape. then. or
perhaps by energetic collisions with other moons.
The ring moons gradually sweep up particles. which are subse-
quently ejected by collisions with large chunks of ring material. The two bright rings. not visible in Figure 22. they are very thin. or coated ice particles that are inefficient reflectors of light. A second small gap (Encke gap) is also visible as a thin
line in the outer portion of the A ring. Planetary geologists think Hyperion’s surface
is so weak and porous that impacts punch into its surface.
Some of the ring particles are believed to be debris ejected
from the moons embedded in them.21 Image taken by the Earth-orbiting Hubble Space Telescope shows Saturn’s dynamic ring system.

Uranus and Neptune:
Twins
Although Earth and Venus have many
similar traits. (Courtesy of NASA/JPL)
Prometheus.25). Their mantles. whereas others possess large. once thought to be weatherless. (Courtesy of NASA/JPL)
formed later.
Uranus. This unusual characteristic of
Uranus is likely due to a huge impact that essentially knocked the
planet sideways from its original orbit early in its evolution. B. Uranus and Neptune are A. Researchers expect
more light to be shed on the origin of
planetary rings as the Cassini spacecraft
continues its tour of Saturn.” They are nearly
equal in diameter (both about four times the size of Earth). water vapor. and organic compounds from
in the A ring. B.
deep canyons and linear scars. and
are both bluish in appearance—a result of methane in their
atmospheres. thin ring.3). Its gravity
helps confine the moonlets in Saturn’s thin F ring. a process called
FIGURE 22.
FIGURE 22.
the area of the tiger stripes.23 NASA’s Cassini Orbiter captured this mosiac of
Saturn’s tectonically active. Their days are nearly the same length and their
Labtayt Sulci cores are made of rocky silicates and iron—similar to the other gas
giants. the fragments of which
would tend to jostle one another and
form a flat. are visible in the lower right. when a moon or large aster-
oid was gravitationally pulled apart after
straying too close to a planet.
Its rotational motion. Some have long. located
jets spurting ice particles.
smooth areas on otherwise crater-riddled surfaces. comprised mainly of water. The northern
hemisphere contains a 1-kilometer deep chasm. It is responsible for keeping the Encke gap open. while linear features. A. One of the most pronounced differences
between Uranus and Neptune is the time they take to complete
one revolution around the Sun—84 and 165 Earth-years. therefore. the innermost of the five largest moons. and
methane. Yet another
hypothesis suggests that a foreign body
collided catastrophically with one of the
planet’s moons. acts as a ring shepard. Recent pho-
tographs from the Hubble Space Telescope also reveal banded
Tiger stripes clouds composed mainly of ammonia and methane ice—similar
to the cloud systems of the other gas giants.
. respec-
tively. are thought to be very different from Jupiter and Saturn
(see Figure 22.24 Two of Saturn’s ring moons. shows evidence of
huge storm systems the size of the United States. icy satellite Enceladus. The find occurred as Uranus passed in
front of a distant star and blocked its view. resembles a rolling ball instead of
a spinning toy top (Figure 22. which lies nearly parallel to the ecliptic (“lying on its side”).
as occurs on Io. a potato-shaped moon. Pan is a small moon
called tiger stripes. ammonia.
Uranus’ Rings A surprise discovery in 1977 showed that
Uranus has a ring system. Jovian Planets 653
perhaps more deserving of being called “twins. Inset image shows
about 30 kilometers in diameter that orbits in the Encke gap.
Uranus’ Moons Spectacular views from Voyager 2 showed that
Uranus’ five largest moons have varied terrains. Studies con-
ducted at California’s Jet Propulsion Laboratory suggest that
Miranda.
Uranus: The Sideways Planet Unique to Uranus is its axis of
rotation. was recently
geologically active—most likely driven by gravitational heating.

500 miles) per hour encircle the planet.400 kilometers (1. and was later gravitationally captured by
Hubble’s Near Infrared Camera. Interspersed among these distinct structures
are broad sheets of dust. More with Neptune in the background.and space-based observations indicate that
the process whereby a solid (ice) changes directly to a gas. Neptune’s
storms appear to have comparatively short life spans—usually
only a few years. This false-color image was generated from data obtained by formed independently.
making Neptune one of the windiest places in the solar system. astronomers knew very little about Neptune until
1989. Still farther south is a second dark spot with a bright core.
of NASA)
Triton and a few other icy moons erupt “fluid” ices—an amaz-
ing manifestation of volcanism. the largest
of which is the moon Triton. Another feature that Neptune has in common
with the other Jovian planets are layers of white.
Neptune has a dynamic atmosphere. the remaining 12 are small.
(Courtesy of NASA/JPL)
FIGURE 22. However. Triton is the only large moon in the solar sys-
tem that exhibits retrograde motion. meaning “frost”) describes the eruption of magmas derived
occultation (occult = hidden).
Neptune’s Moons Neptune has 13 known satellites. accompanied by bright
white clouds. At the top is the Great Dark Spot. much like that of the
other Jovian planets (Figure 22.
tation and again five times afterward (see Figure 22.
Recall that Neptune exhibits large dark spots thought to be rotat-
ing storms similar to Jupiter’s Great Red Spot. cirrus-like clouds
(probably frozen methane) about 50 kilometers above the main
cloud deck. Observers saw the star “wink”
briefly five times (meaning five rings) before the primary occul.26 This image of Neptune was constructed from two
images. FIGURE 22. Twelve years and nearly 3 billion miles of Voyager 2 travel
provided investigators an amazing opportunity to view the out-
ermost planet in the solar system. FIGURE 22. The bottom of the image shows
Triton’s wind and sublimation-eroded south polar cap. irregu-
larly shaped bodies.26).27 This montage shows Triton.27). Neptune’s largest moon. To the south is a bright white area thought to be high
cloud tops. distinct rings orbiting its NASA/JPL)
equatorial region. Record wind speeds that
exceed 2.25 Uranus surrounded by its major rings and 10 of its 27
known moons. Sublimation is
recent ground. (Image by Hubble Space Telescope courtesy Neptune (Figure 22. Cryovolcanism (from the Greek
Kryos. Also visible are cloud patterns and several oval storm
systems. (Courtesy of
Uranus has at least 10 sharp-edged.25). indicating that it most likely
654
.
Neptune: The Windy Planet Because of its great distance
from Earth.

The newest
grouping. which suggests they are com. or occasionally. and three that are narrow.
. ularly pass close to Earth and the Moon (Earth-crossing aster-
oids). and many millions that are smaller. Images obtained as the spacecraft drifted toward the sur-
ring systems? face of Eros revealed a barren.29). on the other hand. and are the orbits of a few known near-Earth asteroids. Many of the recent large-impact craters on the Moon and
Neptune’s Rings Neptune has five named rings. most researchers agree that
CONCEPT CHECK 22. Triton’s icy Asteroids: Leftover Planetesimals
magma is a mixture of water-ice. dwarf planets. ice lavas develop that can flow great distances from eccentric orbits that take them very near the Sun. comets. the
combined mass of all asteroids is now estimated to be only 1/1. Earth–asteroid col-
fined by the satellite Galatea. what happens when a jar of sand and various sized pebbles is
ing the eight planets and in the outer reaches of the solar system.
One of several hypotheses to explain the boulder-strewn
Small Solar System Bodies topography is seismic shaking. When partially melted. like “piles of rubble. Today. more than 1 kilometer in diameter. and probably ammo-
nia. pro. one-third of which are
kilometers wide. They are Mars
distinguished according to size: Asteroids are Earth
larger than 100 meters in diameter. Researchers unexpectedly discovered that
) Name three bodies in the solar system that exhibit active fine debris tends to concentrate in the low areas where it forms flat
volcanism. Voyager 2 detected five that are more than 400 kilometers in diameter. this mixture behaves as molten rock Asteroids are small bodies (planetesimals) remaining from the
does on Earth. Some travel along
environments. shaken—the larger pebbles rise to the top while the smaller sand
In 2006. There are only
erate the ice equivalent of volcanic ash. Although it was not designed for landing. planet that once orbited between Mars and Jupiter.6 billion years
can generate quiet outpourings of ice lavas. the
Asteroid belt
largest known object in the asteroid belt. two of which
Earth were probably the result of collisions with asteroids. whereas
meteoroids have diameters less than 100 meters. upon reaching the surface these magmas formation of the solar system. deposits resembling ponds. methane. suggesting they
" What is the nature of Jupiter’s Great Red Spot?
are porous bodies.
Comets. which would cause the boulders
to move upward as the finer materials sink. About
are broad. In 1989. Because most asteroids have irregular shapes.
Jupiter’s in that they appear faint. a former planet. Neptune’s rings also display red ogists initially speculated that they might be fragments of a broken
colors that indicate the dust is composed of organic compounds.28). Neptune’s rings are most similar to lisions will occur again (Box 22. the land-
scape is marked by an abundance of large boulders. rocky surface composed of particles
( How are Saturn’s satellite Titan and Neptune’s satellite Triton ranging in size from fine dust to boulders up to 10 meters (30 feet)
similar? across (Figure 22. In other meter.
meteoroids and (2) dwarf planets. and small rocky particles that origi-
nate in the outer reaches of the solar system. An explosive eruptive column can gen. planetary geol-
posed mostly of dust-size particles. making them about 4. dust. but the solar
active plumes on Triton that rose 8 kilometers above the surface system hosts an estimated 1–2 million asteroids larger than 1 kilo-
and were blown downwind for more than 100 kilometers.
system objects not classified as planets or moons
into two broad categories: (1) small solar system FIGURE 22. region known as the asteroid belt (Figure 22. Surrounding the low areas. and others reg-
their source—similar to the fluid basaltic flows on Hawaii. includes Ceres.5 asteroids are leftover debris from the solar nebula.
Asteroids and meteoroids are composed of
rocky and/or metallic material with compositions
somewhat like the terrestrial planets. and
Pluto. Asteroids have
lower densities than scientists originally thought.
been captured? NEAR—Shoemaker landed successfully on Eros and collected
& How are Jupiter and Saturn similar? information that has planetary geologists both intrigued and per-
' What two roles do ring moons play in the nature of planetary plexed. The outermost ring appears to be partially con. the International Astronomical Union organized solar grains settle to the bottom. Small Solar System Bodies 655
from the partial melting of ice instead of silicate rocks.1). are loose collections Jupiter
of ices. In fact. Most asteroids orbit the Sun between Mars and Jupiter in the
duce explosive eruptions. Also shown
bodies that include asteroids. perhaps no more than 100 2. This is analogous to
There are countless chunks of debris in the vast spaces separat. old. However.28 The orbits of most asteroids lie between Mars and Jupiter. Inevitably.000
of the modest-sized Earth.” loosely bound together.
# Why are the Galilean satellites of Jupiter so named?
$ What is distinctive about Jupiter’s satellite.000 Earth-crossing asteroids are known. Io?
In February 2001 an American spacecraft became the first vis-
% Why are many of Jupiter’s small satellites thought to have itor to an asteroid.

a “fireball” that Earth objects. The lake outlines the crater
satellite images became available kilometers above the surface.
active comets. (Courtesy of U. expeditions to 1. and extinct comets. Additional structures are being identified
ingly clear that comets and asteroids collide
every year. In November the solar system. thus the nickname “dirty snowballs.
across.” Statistics show that colli-
have caused the demise of the dinosaurs. ters (44. produced a crater 10 kilometers (6 miles)
oid about 10 kilometers (6 miles) in diameter wide. is a
Most impact structures are so old and highly the explosion.A World map of major impact structures.2 miles)
collided with Earth off the Yucatan Penin. and perhaps 2 kilometers (1. A few large
asteroids may have completely melted. methane.
. As an observer noted. Called the them toward Earth. the Japanese probe Hayabusa landed on a small near. They are loose collections of rocky material. a large aster. Surprisingly. water ice. about the nearly 1 kilometer across shot past Earth in
size of an Olympic swimming pool (50 a “near miss.000 kilometers away. indicates their ices are hidden beneath a rocky layer.B). it scorched. causing them to differ. are leftover material from the formation of
entiate into a dense iron core and a rocky mantle. known as Meteor Crater (see but deadly objects from space came to pub-
kilometers. dust. their orbits may be altered by the
windows and triggered reverberations heard gravitational interaction.
FIGURE 22. This crater was pro.000 miles) per hour.000 have been classified as potentially
the area found no evidence of an impact hazardous asteroids. a spectacular explosion extinctions. larger than 6 kilometers. which may send
up to 1. page 661). like asteroids.” Traveling at 70. Why it
remnant. As of December 2010.) crater or any metallic fragments. and frozen gases (ammonia. it could have
About 65 million years ago. Comets. in NASA scientists continually track near-
a remote region of Siberia.S. deep.000 kilome-
meters in diameter). These
fragments travel at great speeds and can
strike Earth with an explosive force many
times greater than a powerful nuclear
weapon. as evidenced by the many
large impact structures that have been iden-
tified (Figure 22. (Many impact craters
were once mistaken for volcanic structures.
Tunguska event.
2005. lic attention again in 1989 when an asteroid
duced by a relatively small body.1
EARTH AS
A SYSTEM
Is Earth on a Collision
Course?
The solar system is cluttered with asteroids.
In recent decades. and more than 7000 near-Earth objects have
flattened trees up to 30 kilometers from the been discovered.B Manicouagan. Collisions with bodies
Chapter 12).
extend outward for an additional 30
Arizona. “Sooner or later
sula in Mexico. of which slightly more than
point of impact. Fractures related to this event
very fresh-looking crater near Winslow.
Indirect evidence from meteorites suggests that some aster.33.” Recent space missions
June 2010. delimbed. When asteroids or comets
appeared more brilliant than the Sun pass closely to any large body in the solar
exploded violently. which
collected. sions with bodies larger than 1 kilometer
as well as the extinction of nearly 50 percent should be expected every few hundred
of all plant and animal species (see thousand years. In 1908.656 CHAPTER 22 Touring Our Solar System
Box 22. Comets: Dirty Snowballs
oids might have been heated by a large impact event.
asteroid or comet with our planet. resulting in mass
More recently. Geological Survey)
Figure 22. The shock waves rattled system. it has become increas-
FIGURE 22. which is 70 kilometers (42 miles)
(Figure 22. are anticipated every 100 million
has been attributed to the collision of an years. occurred several
structure. Quebec. (Data from Griffith Observatory)
with Earth far more frequently than previ-
ously thought. and carbon diox-
Earth asteroid named 25143 Itokawa and returned to Earth in ide). One notable exception is a exploded prior to impact is uncertain. This impact is thought to it will be back. which equaled at least a 10-
200-million-year-old eroded impact
eroded that they were not discovered until megaton nuclear bomb. However. Evidently. The dangers of living with these small. it remains uncertain if samples were to comets have shown their surfaces to be dry and dusty.A).

Inset shows a close-up of Eros’s barren rocky returns to cold storage.
As a comet’s orbit carries it away from the Sun. a smaller number of short. (Courtesy of NASA) to form the tail is lost forever. page 637). Laboratory studies
shortest period comet (Encke’s Comet) orbits around the Sun revealed that the coma contained a wide range of organic com-
once every three years. Images from Stardust show that the comet’s
years). most develop tails that can
extend for millions of kilometers. One is radiation pressure caused by radiant energy (light)
emitted by the Sun. Sometimes a single tail
composed of both dust and ionized gases is produced. a stream of
charged particles ejected from the Sun.30 Orientation of a comet’s tail as it orbits the Sun.
and take hundreds of thousands of years to complete a single The very first samples from a comet’s coma (Comet Wild 2)
orbit around the Sun. make regular surface was riddled with flat-bottomed depressions and appeared
encounters with the inner solar system (Figure 22. most comets are too
Fully small and too distant to be
formed. solar energy
begins to vaporize their ices.30).30). A chance collision
to form between two Kuiper belt comets. The heavier dust particles
produce a slightly curved tail that follows the comet’s orbit. When comets reach the inner solar system.
This disc-shaped structure is
thought to contain about a billion
objects over 1 kilometer in size.32). most Kuiper belt comets
move in slightly elliptical orbits
Orbit that lie roughly in the same plane
Dust tail
beginning Ion tail as the planets. These structures are typically 1–10
kilometers in diameter. who
Tail of ionized gases predicted its existence. However.
or the gravitational influence of
. The dry.
Most comets originate in one
of two regions: the Kuiper belt or
FIGURE 22. and the comet
Shoemaker probe.
curved observed from Earth. the tail disappears. It is believed that few comets
Most comets reside in the outer reaches of the solar system remain active for more than a few hundred close orbits of the Sun. beyond Nep-
tune (see Figure 22. The escaping gases carry dust from the
comet’s surface. producing a highly reflective halo called a coma
(Figure 22. The tail of a comet points away
from the Sun in a slightly curved manner (see Figure 22. Within the coma.
Tail composed of dust However. Small Solar System Bodies 657
All of the phenomena associated with comets come from a small
Close-up of surface central body called the nucleus.30). were returned to Earth in January 2006 by NASA’s Stardust space-
period comets (those having orbital periods of less than 200 craft (Figure 22. Named in honor of
astronomer Gerald Kuiper.29 Image of asteroid Eros obtained by the NEAR-
ing the coma recondense.1. the
inactive comet. Like
Sun the asteroids in the inner solar sys-
tem. Scientists
have identified two solar forces known to contribute to tail for-
mation. such as the famous Halley’s Comet. Material that was blown from the coma
surface. the Kuiper
belt hosts comets that orbit in the
outer solar system.
the Oort cloud. although at least 10 gas jets were active. the small glowing nucleus with a
diameter of only a few kilometers can sometimes be detected.
As comets approach the Sun. pounds and substantial amounts of silicate crystals. even using
dust tail the Hubble Space Telescope. but two
tails are often observed (Figure 22.
whereas the extremely light ionized gases are “pushed” directly
away from the Sun. which
led early astronomers to believe that the Sun has a repulsive force
that pushes away particles of the coma to form the tail. which closely resembles an asteroid. but nuclei 40 kilometers across have been
observed. continues
its orbit without a coma or tail. forming the second tail. When all the gases are expelled. and the second is the solar wind.31). the gases form-
FIGURE 22.

and every one of its 29 appearances since 240 Before Apollo astronauts brought Moon rocks back to Earth. only a tiny frac-
tion of Oort cloud comets have orbits that bring them into the
inner solar system. artist’s concept depicting jets of
gas and dust erupting from Comet Wild 2. pro-
FIGURE 22. Most
notable is Arizona’s Meteor Crater. However.
forming a spherical shell around the solar system. has been recorded by Chinese astronomers—testimony to
meteorites were the only extraterrestrial materials that could be
their dedication as astronomical observers and the endurance of studied in the laboratory. created by friction between the meteoroid and the air. More than 250 others may be of impact origin. or can last as “long” as
a few seconds. (2) material that is continually
being ejected from the asteroid belt. called micro-
meteorites. Most meteoroids
nucleus of the comet is within the bright spot in the center. Earth’s gravity does the rest. where chance collisions or gravitational
interactions with Jupiter modify their orbits and project them
toward Earth. Its orbital period Mars. Named for Dutch astronomer Jan Oort. are so tiny and their rate of fall so slow that they drift
to Earth continually as space dust. They occur when a small solid particle. Most Oort cloud
comets orbit the Sun at distances greater than 10. was uncharacteristically
originate from one of the following three sources: (1) interplan-
active during its most recent entry into the inner solar system. or (3) the rocky and/or
one of the Jovian planets.
Halley’s Comet originated in the Kuiper belt. Inset.
averages 76 years. In 1910. The duces the light we see trailing across the sky.32 Comet Wild 2. or possibly Mercury. At least 40 ter-
restrial craters exhibit features that could only be produced by an
explosive impact of a large asteroid. dark night. (Courtesy of NASA) with the naked eye anywhere on Earth. courtesy of NASA) ing formation of the solar system. A few meteoroids are probably fragments of the Moon. orbit. commonly (but inaccu-
rately) called “shooting stars. result when
Earth encounters a swarm of meteoroids traveling in the same
direction at nearly the same speed as Earth.2).
Heat. which orbits the Sun every 6 years.
Chinese culture. metallic remains of comets that once passed through Earth’s
ciently to send them into our view. called meteor showers. a
meteoroid.000 times the
Earth–Sun distance. are probably the scattered remains of the nucleus
of a long-defunct comet. The notable Perseid meteor shower that
occurs each year around August 12 is likely material ejected from
the comet Swift-Tuttle on previous approaches to the Sun.
sunset on a clear. Comet
Holmes. as seen by NASA’s Stardust spacecraft. enters Earth’s atmosphere from interplanetary space.C. a huge cavity more than 1 kilo-
658
. making for a spectacular display. not associated with the orbits of
known comets.
A few very large meteoroids have blasted craters on Earth’s
surface that strongly resemble those on our Moon. the Oort cloud con-
sists of comets that are distributed in all directions from the Sun. meteor sightings increase dramatically to 60 or
more per hour. Halley’s Comet made a very close Meteoroids less than about a meter in diameter generally
approach to Earth.
Occasionally. (Image etary debris missed by the gravitational sweep of the planets dur-
by Spitzer Space Telescope. The close association
of these swarms to the orbits of some short-term comets strongly
suggests that they represent material lost by these comets
(Table 22. many are bright enough to be seen
during its flyby of the comet.” These streaks of light can be
observed in as little as the blink of an eye. vaporize before reaching Earth’s surface. ejected by a violent asteroid impact.
Meteoroids: Visitors to Earth
Nearly everyone has seen a meteor. Some.
B. occasionally alters their orbits suffi. After
FIGURE 22.
Most meteoroids large enough to survive probably originate
among the asteroids. Some swarms.31 Coma of comet Holmes as it orbits the Sun. or perhaps even a comet
nucleus. Researchers estimate that thou-
sands of meteoroids enter Earth’s atmosphere every day. The gravitational effect of a distant passing
star may send an occasional Oort cloud comet into a highly eccen-
tric orbit that carries it toward the Sun. These displays.

This “old age”
impact likely occurred within the last 50. In addition.33). has been confirmed by data obtained from lunar samples. which are the basic building blocks of life. If mete-
orites represent the composition of the terrestrial planets. Classified by their composition. silicate minerals with
Lyrids April 20–23 Comet 1861 I inclusions of other minerals. the age of our solar system is about 4. which indi-
Geminids December 4–16
cate that numerous organic compounds exist in interstellar space.
Data from meteorites have been used to ascertain the inter-
nal structure of Earth and the age of the solar system. weather more slowly. Small Solar System Bodies 659
TABLE 22.34). This cavity is about 1. are referred
to as meteorites (Figure 22. mixtures of the
Eta Aquarids May 3–5 Halley’s Comet two. (Photo by Michael Collier)
. and are easily distinguished
Draconids October 7–10 Comet Giacobini-Zinner from terrestrial rocks. as some
meter wide.2 Major Meteor Showers The remains of meteoroids.
FIGURE 22. is one reason that geologists think Earth’s core is mostly iron and
but attempts to locate the main body have been unsuccessful. near Winslow. The
solar system is cluttered with meteoroids and other objects that can strike Earth with explosive force.6 billion years. mostly iron with 5–20 percent
Quadrantids January 4–6 nickel.75 mile) across and 170 meters (560 feet) deep. irons are
DeltaAquarids July 30 found in large numbers because metallic meteorites withstand
Perseids August 12 Comet 1862 III impacts better.
contains organic compounds and occasionally simple amino
Andromedids November 14 Comet Biela
acids. More than larger percentage of iron than is indicated by surface rocks. nickel. 170 meters (560 feet) deep. or (3) stony–irons.2 kilometers (0. Iron meteorites are probably fragments of
Orionids October 20 Halley’s Comet once molten cores of large asteroids or small planets. with an upturned rim that planetary geologists suggest. This discovery
Leonids November 18 Comet 1866 I
confirms similar findings in observational astronomy. Arizona. (2) stony (also called chondrites). Although stony meteorites are the most common.000 years. This
30 tons of iron fragments have been found in the immediate area. radiometric dating of meteorites indicates the
Based on the amount of erosion observed on the crater rim. our planet must contain a much
rises above the surrounding countryside (Figure 22. called a carbonaceous chondrite.33 Meteor Crater.
Taurids November 3–13 Comet Encke
One type of stony meteorite.
Shower Approximate Dates Associated Comet
meteorites are either (1) irons. when found on Earth.

Astronomers continued to discover dozens of other
ery. Pluto is rec-
ognized as a dwarf planet and the prototype of this new category
of planetary objects. At the time of its discov. These are celestial bodies that orbit the Sun. was launched in January 2006. standing the solar system. Most recently. Arizona. including Earth’s.FIGURE 22. a Kuiper
Dwarf Planets belt object. indi. seven moons in the solar tremendous potential for aiding researchers in further under-
system. Researchers now recognize that Pluto was unique among the
cated that it was less than half Earth’s diameter. ical due to their own gravity. Scheduled to fly by Pluto in
diameter of Earth and less than half that of Mercury (long con. and later explore the Kuiper belt. the largest known asteroid. Pluto has been a mystery to the mid-1800s.
Even more attention was given to Pluto’s status as a planet
when astronomers discovered another large icy body in orbit
beyond Neptune. Ceres.
explain irregularities in Neptune’s orbit. Image by the
Hubble Space Telescope. including the asteroids Vesta. are larger than Pluto.
(Courtesy of Meteor Crater Enterprises. and Pallas.35 Pluto with its three known moons. Inc. are essentially spher-
meteor.430 miles).7
" Define dwarf planets.300 kilometers (1. astronomy textbooks listed as many as 11 planets
astronomers who were searching for another planet in order to in our solar system. are thought to exist in this belt of icy
$ What do you think would happen if Earth passed through the worlds beyond Neptune’s orbit. By this definition. and meteorite.
tail of a comet? The International Astronomical Union. but located at the outskirts of the solar system. In fact. in 1978.” a clear signal that these small bodies represent a class
nificantly alter Neptune’s orbit.
& Differentiate between the following solar bodies: meteoroid. Other dwarf planets include Eris.)
FIGURE 22. about one-fifth the tem. Then. adjusted because of improved satellite images. New Hori-
images obtained by the Hubble Space Telescope show that Pluto’s zons. classical planets—completely different from the four rocky. The new classi-
really is because of the brightness of its newly discovered satellite.35).” In
Since its discovery in 1930.6
" Where are most asteroids found?
# Compare and contrast asteroids and comets.
asteroid belt. July 2015. fication will give a home to the hundreds of additional dwarf
Charon (Figure 22.
perhaps larger than Pluto. Pluto was thought to be the size of Earth—too small to sig. some
660
. inner-
astronomers realized that Pluto appeared much larger than it most planets. and Ceres.
Pluto’s reclassification was not the first such “demotion. the group responsi-
% Where are most comets thought to reside? What eventually ble for naming and classifying celestial objects. Juno. calculations based on planets astronomers assume exist in the solar system. as well as the four gaseous giants. Later. but are not large enough to sweep
' What are the three main sources of meteoroids? their orbits clear of other debris. voted in 2006 to
becomes of comets that orbit close to the Sun? create a new class of solar system objects called dwarf planets. estimates of Pluto’s of objects separate from the planets. Many other planetary objects. the first spacecraft designed to explore the outer solar sys-
diameter is 2. New Horizons carries
sidered the solar system’s “runt”).34 Iron meteorite found near Meteor Crater. over a thousand of these Kuiper belt
objects were discovered forming a band of objects—a second
CONCEPT CHECK 22. The # Why was Pluto demoted from the ranks of the classical
Kuiper belt objects are rich in ices and have physical properties planets?
similar to those of comets. Soon. (Courtesy of NASA)
CONCEPT CHECK 22.
diameter. “planets.

C. Using Table 22.390. and D). The accompanying diagram shows two of Uranus’ moons.
2. approximately how many Earths would
fit side-by-side between Earth and the Moon?
c.
5.756 km)?
b. The accompanying table shows data that have been
gathered about three of the planets orbiting the central star of this newly
discovered solar system.798 km. terrestrial. In order to conceptualize the size and scale of Earth and Moon as they relate to the solar system.474 km) would fit side-by-side across the
diameter of Earth (diameter = 12. Dwarf Planets 661
GIVE IT SOME THOUGHT
1. Approximately how many Moons (diameter = 3. Use it to com-
plete the following. Approximately how many Suns would fit side-by-side between Earth and the Sun. Explain what would happen to the Epsilon ring if a large
asteroid struck Ophelia.000. Approximately how many Earths would fit side-by-side across the Sun whose diameter is about
1. knocking it out of the Uranian system. Ophelia and Cordelia. Crater D has one secondary crater (labeled
“d”). B. Imagine that a solar system has been discovered in a nearby region of the
Milky Way Galaxy. The accompanying graph shows the temperature at various distances
from the Sun during the formation of our solar system.
. The accompanying sketch shows four primary craters (A.
complete the following.1 as a guide. which act as
shepherd moons for the Epsilon ring. Explain your reasoning.
The impact that produced crater A produced two secondary craters
(labeled “a”) and three rays.000 km?
3. Given that the Moon’s orbital diameter is 768. classify each planet as
either Jovian.
a. Which planet(s) formed at locations where the temperature in the
solar system was cooler than the freezing point of water?
4. Which planet(s) formed at locations where the temperature in the
solar system was hotter than the boiling point of water?
b.000 km?
d. or neither. Rank the four primary craters from oldest to youngest and explain
your ranking.
a. a distance of
about 150.

methane. (2) excavations of the large impact that never accreted into a planet. Recently. and (2) dwarf planets. The accompanying diagram shows a comet traveling toward the Sun at the first position where
it has both an ion tail and a dust tail. With
an orbital period of 76 years. Saturn is best known for its system of rings. one
tail.
. Another friend argues
that the objects should be classified as dwarf planets.662 CHAPTER 22 Touring Our Solar System
6. and vaporize with a flash of light. comets. and (4) formation of youth. more dense. become meteors when they enter Earth’s atmosphere
surface of relatively subdued plains and inactive volcanic fea. It also
ger atmospheres. State whether you agree or
disagree with either or both of your friends. the International Asronomical Union organized solar
lunar regolith. Would your answers to the preceding question change if the Sun’s energy output were to
increase significantly? If so. Earth encounters a swarm of meteoroids. or two tails. Scientists conclude that the asteroids lie between the orbits of Mars and Jupiter. the terres. probably material
and surface temperatures of 475° C (900° F). has a thick. Comets are made of ices
basins. rotates rapidly. assumed to be large rotating storms similar to
layer of very fluid basaltic lava.
craters were produced by the collision of rapidly moving Uranus and Neptune are often called “the twins” because of
interplanetary debris (meteoroids). the largest planet.
with a soil-like layer of gray. a Great Red Spot that varies in size. Earth.
" Mercury is a small. Venus. called " In 2006. is perhaps the most volcanically active body in the solar
more rocky material. has a banded
(Earth-like) planets (Mercury. (3) filling of maria basins. One of your friends argues the objects
should be classified as planets because they are large and orbit the Sun. One hypothesis for the broad catergories: (1) small solar system bodies that include
Moon’s origin suggests that a Mars-sized object collided with asteroids. densely cratered their similar structure and composition. Maria basins are enor. extensive dust storms. which has been derived from a few billion system objects not classified as planets or moons into two
years of meteroic bombardment.
a.
volcanoes. Explain your reasoning. All lunar terrains are mantled Jupiter’s Great Red Spot. a thin ring system. system). Bright. Assume three irregularly shaped planet-like objects. Pluto was placed into a new class of
dense as Earth’s. contain proportionally Io. Meteor showers occur when
tures.
trial planets are smaller. It has been estimated that Halley’s Comet has a mass of 100 billion tons. Saturn. fairly Uranus is the fact that it rotates on its side. cirrus-like clouds above its main cloud deck and large
mous impact craters that were later flooded with layer upon dark spots. monoxide) with small pieces of rocky and metallic material. oroids. and carbon
ful rayed craters. have just
been discovered orbiting the Sun at a distance of 35 AU. unconsolidated debris. Uranus. how would it affect this comet and its tails?
7. and several valleys exhibiting
drainage patterns similar to stream valleys on Earth. Impact per hour and storms similar to Jupiter’s Great Red Spot. and mea. numerous inactive solar system objects called dwarf planets. If the solar wind suddenly ceased. Comets are thought to reside in the outer solar system in one
sphere and exhibits the greatest temperature extremes of any of two locations: the Kuiper belt or the Oort cloud. each smaller than our Moon. has a carbon dioxide atmosphere only 1 percent as found on Earth. Asteroids
lunar surface evolved in four stages: (1) formation of the origi.
8. The dark. and Mars) and appearance caused by huge convection currents driven by
the Jovian (Jupiter-like) planets (Jupiter. (water. it is esti-
mated to lose about 100 million tons of material when its orbit brings it close to the Sun. If one tail or two tails are present.
Neptune). have slower rates of rotation. a space.
In Review Chapter 22 Planetary Geology
" The planets are arranged into two groups: the terrestrial " Jupiter. the Red lost by a comet. Furthermore. such as Pluto. and at least 63 moons (one of the moons. and the planet’s interior heat. a surface atmospheric pressure 90 times that of Earth’s. carbon dioxide. indicate whether the comet will have no tails. the brightest planet in the sky. Most
Earth to produce the Moon. small solid particles that travel through interplanetary
heavy atmosphere composed of 97 percent carbon dioxide. Mete-
planet. Neptune has
smooth lowlands are called maria. how would they change?
c. Meteorites are the remains of meteoroids
Planet. many large canyons. white. are leftover rocky and metallic debris from the solar nebula
nal crust (highlands). Mars. For each of the three numbered sites. meteoroids. has a dynamic atmosphere with strong winds up to 930 miles
" The lunar surface exhibits several types of features. ammonia. Refer to this diagram to complete the following. dense planet that has virtually no atmo. in what direction will they point?
b. A unique feature of
highlands make up most of the lunar surface. calculate the maximum remaining life span of Halley’s Comet. Venus. When compared to the Jovian planets.

com to find practice news articles are pulled into the site with assessment
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illuminating many difficult-to-understand Earth science
concepts
Looking for additional review and test prep materials? Visit the • In The News RSS Feeds: Current Earth science events and
Self Study area in www. on the Moon? 3. 655) lunar highlands (p. 642) Oort cloud (p. 640) meteorite (p. and gas) might influence the biosphere. hydrologic cycle be different if (a) its orbit was inside the
sional influences from our near-space neighbors. • Pearson eText
standing of this chapter’s content. 637)
cryovolcanism (p.
Because the Moon lacks these spheres. or nearly absent. and multimedia that will aid in your under. and biosphere) interact as a system with occa. 657) nucleus (p. 636)
comet (p. found on Mercury. 637)
Jovian planet (p. liquid. 643) planetesimals (p. 657)
asteroid belt (p. 654) meteor (p. Mastering Geology 663
Key Terms
asteroids (p. 658) terrestrial planet (p. 656) maria (p. 658) solar nebula (p. 655) meteor shower (p. 636)
Examining the Earth System
1. On Earth the four major spheres (atmosphere. speculate about how the changes
because water exists in all three states (solid. Among the planets in our solar system. 655) Kuiper belt (p.masteringgeology. 658)
coma (p. Venus. 640) meteoroid (p. hydrosphere. study tools.
on and near its surface. 637) Nebular theory (p. 655)
dwarf planet (p. In MasteringGeology™ you • Optional Self Study Quizzes
will find: • Web Links
• GEODe: Earth Science: An interactive visual walkthrough of • Glossary
key concepts • Flashcards
. 636)
escape velocity (p. 642)
impact craters (p. list at least five what effect might this event have on the atmosphere (in par-
processes that operate on Earth but are absent on the Moon. 658) small solar system bodies (p. Earth is unique conditions persisted. 659) terrae (p. If a large meteorite were to strike Earth in the near future. average temperatures and climate)? Assuming the
2. Which of orbit of Venus? (b) its orbit was outside the orbit of Mars?
these spheres are absent. and Mars? How would Earth’s
geosphere. 657) lunar regolith (p. ticular. 642) protoplanets (p.